
This means that \(J\)  and \(\tfrac{1}{2} (\tilde{\rho }_2-\tilde{\rho }_1) J_\phi \)  can merely differ by an additive source-free function. As in [1]}, we thus assume that
\(J= \frac{\tilde{\rho }_2-\tilde{\rho }_1}{2} J_\phi \quad \text{in $Q$.}\) 
